Constant HEADROOM_WIDTH 

const HEADROOM_WIDTH: u32 = 4096;

The geometry every session is initialized to hold, so reconfigure_resolution can grow into it in place. It spans both UHD and DCI 4K and is the largest picture the 5.x levels of all three codecs admit, which is what lets AV1 – pinned to this level – stay inside a level a decoder accepts; a taller resize rebuilds the session instead.